Most of modern mass data recording techniques with writing/reading head movable relative to a recording layer utilize spatial variation in magnetic property or optical property in the recording layer. Some other recording techniques utilize variation of electrical polarization or deposition of static electric charge. Many memory devices utilize change of electrical resistance of some semiconductor or dielectric films, but few mass data recording techniques with writing/reading head movable relative to a recording layer utilize variation of electrical impedance.
As far as we know, one patent, U.S. Pat. No. 3,511,973, which is issued in 1970 to Donald T. Best, aims to utilize impedance change for data recording layer movable relative to some writing/reading apparatus. In this patent, the recording layer is a piece of card, rectangular in shape, having a uniform surface resistivity. On two opposite sides of this rectangular card there are two metal strips in contact with the resistive surface. Data is recorded by altering the resistivity of a data spot in the medium by means of mechanical impact, hot probe, application of a high potential, chemical means or the like. Data is retrieved by applying a potential across the metal strips, creating a nearly uniform current density along the surface of the recording layer and causing the temperature at the surface to rise. At those spots on the recording layer where the resistivity has been changed, the temperature is different from other areas as a whole. The temperature difference is then detected by a scanning thermographic equipment.
There are several disadvantages in Best's patent. The reading current proposed by this patent flows through a series of data spots in the recording layer. As a result, resistance change of one data spot would influence the current flow and the temperature in the data spots in series with this data spot. For example, if the local resistance of a data spot is increased, the data spots in series with it would become cooler because of decrease of current flowing through them, even though there are no resistance changes in these spots. Furthermore, because of thermal conductivity, temperature change in one spot will influent the temperature in its neighborhood. All these make the fine detailed analysis of the data structure difficult.